1. Field of the Invention
This invention relates to the field of chromatography and more particularly, to improvements in chromatographic columns used as an analytical tool for the separation, identification, and quantitative determination of mixtures of volatile compounds (gases and liquids).
2. Description of the Prior Art
Chromatography, in all forms, is a technique used to separate a mixture of compounds or elements into the components thereof. All chromatographic processes consist of two basic segments: a mobile phase and an immobile phase. The mobile phase moves through a tube, called a column containing the immobile phase, and the sample to be separated into its basic components is injected in the mobile phase into the column. As the sample is swept forward by the mobile phase, the sample components are either adsorbed on the surface of the immobile phase (if the immobile phase is a solid) or dissolved in the immobile phase (if it is a liquid). As the mobile phase passes into the column behind the sample, components of the sample desorb back into it. This adsorption-desorption process continues throughout the length of the column. Each sample component in the mobile phase moves at a different rate through the column depending on its attraction for the immobile phase. The components therefore separate as they pass through the column and emerge at the other end of the column at different times.
In gas chromatography, the sample to be analyzed is comprised of volatile components which are carried through the column in a gaseous state by an inert mobile phase, called the carrier gas. The immobile phase is either a solid in the form of uniform particles or a thin film on either uniform particles or the column walls.
Therefore, under proper conditions, various components of the gas sample are spacially separated by the process of selective adsorption and desorption so that the separated gas constituents issue from the end of the column in sequential order corresponding to their relative volatility, their molecular weight, or other property affecting the degree of adsorption on the immobile phase or packing material in the column. Conventionally, as the separated gases emerge they are passed through a suitable detector element which measures a property of the gas indicative of the character and amount present.
The immobile phase or packing most commonly used in chromatographic columns comprise diatomaceous earth, alumina, glass beads, fluorocarbons, and silica gel. Conventionally the packing, whatever form is chosen for a particular chromatographic column, is poured into the column in granular form and compacted therein by vibration, tamping or the like. An inert gas, such as helium, argon or nitrogen acts as the carrier gas for the sample and flows continuously through the instrument. The use of an inert mobile carrier gas assures that the carrier gas does not react with either the sample or the immobile phase.
Samples are introduced into the carrier gas either as a liquid or a gas. Usually, liquid samples in the order of 10 microliters or less are injected rapidly into a chamber which is maintained at a temperature that insures quick and complete vaporization of the sample.
The efficiency of the column determines the length of time it takes in order to perform an analysis. That is, a column of high efficiency can be of shorter length than a column of low efficiency and with the carrier gas flowing at the same rate, the analysis can be formed much more readily. It is, therefore, desirable to improve the efficiency of a column for two principal reasons; the first, is that a column of high efficiency can perform an analysis much more rapidly, and the second being that a column with high efficiency of an equivalent length compared to a column of standard efficiency will have greater resolution or the ability to measure the relative separation of two sample components and hence the capability of analyzing products, more precisely.
Efficiency of chromatographic columns is expressed in terms of theoretical plate height, which is simply a number of theoretical plates per unit of length necessary to effect resolution. As a component in a sample is moved through the column by a carrier gas, the velocity at which the component is travelling, the dimensions of the column, and the medium through which it travels will have a direct influence on column efficiency.
Eddy diffusion of the component around the column packing material and within the carrier gas will result in a loss of efficiency. Therefore, in order to minimize diffusion, highly efficient packed columns require (1) a solid supportive small, uniform particle size having high surface area or liquid phases of low viscosity on maximum loading; (2) column dimensions of longest practical length and smallest practical diameter, and (3) operation at the optimum flow rate of the carrier gas. Assuming that the carrier gas is injected at the optimum flow rate, the type of packing and column dimensions are the critical criteria for obtaining high efficiency.
Normally, chromatographic columns are made of either glass or metal tubing of convenient length and diameter. Glass has a principal advantage over metal primarily in its non-reactiveness with any of the components which may be used either in the mobile or immobile phases. Glass columns have generally been manufactured of capillary tubing having internal diameters of from 0.25mm to larger than 1mm with outside diameters from 4mm to 6mm. Because of the nature of the glass tubing to be used and fabricated, it is extremely difficult to fabricate columns of lengths greater than 25 feet. The possibility of inadvertent scratching or abrasion of a small section of the capillary tubing, as well as the combination of thermal change and vibrational stresses induced upon the column increase the possibilities of fracture. Therefore, while it is desirable to have columns as long as 200 feet manufactured of glass of small diameter tubing so as to increase the efficiency of the column, or simply, or more durable column, it has been nearly impossible to fabricate such a glass column.
This invention relates to a technique for fabricating such a glass column and to the resultant column with the attendant results of increasing efficiency in such columns from a conventional 800 plates per foot to in excess of 2000 plates per foot.